This invention relates to an apparatus and method for deep-drawing a compartment tray having ridges separating the various compartments from paperboard stock. More particularly, this invention relates to the formation of a selected compartment of a multi-compartmented tray using a male pre-forming die and a separate male finish-forming die.
In the art of constructing multi-compartmented trays by drawing paperboard or the like in a press, it has been difficult to draw trays without tearing, rupturing, or otherwise damaging the interior walls of the tray compartments and the common regions interconnecting adjacent tray compartments. One cause of this manufacturing problem is the inability of paperboard captured between a male die and an interior wall of a female mold cavity during an initial stage of a compartment-forming sequence to flow along the interior wall during subsequent drawing operations. While the paperboard forming the bottom wall of the tray compartment is substantially unaffected or unstressed by the drawing operations, the paperboard used to form the tray compartment side walls is held under a predetermined tension during the drawing process. Occasionally, certain portions of this taut drawn paperboard are stretched beyond the elastic limit of the paperboard material during drawing operations resulting in paper rupture or tearing.
One object of the present invention is to provide a die system for varying the pressure applied to paperboard advanced into a mold cavity during a molding cycle so that paperboard is allowed to flow in the mold cavity sufficiently to relieve stress concentrations developed in the paperboard during paperboard drawing operations associated with the molding cycle.
Another object of the present invention is to provide a die system for distributing paperboard that has been pre-loaded into a selected mold cavity of a female die assembly along an interior wall of the cavity toward paperboard drawing regions of the female die assembly adjacent to the cavity as additional paperboard is being drawn into the cavity under tension. Such distribution advantageously acts to augment or bolster the supply of paperboard material in stress regions that would otherwise be stretched beyond their elastic limit during drawing operations, thereby reducing the likelihood of paper rupture or tearing in those stress regions.
Yet another object of the present invention is to provide a die system including a male die assembly having a pre-forming die that is operable to allow substantially free flow of paperboard material between the pre-forming die and the interior walls of a selected mold cavity formed in a companion female die during paperboard drawing operations associated with finish-forming a tray compartment in the selected mold cavity using a separate finish-forming die.
According to the present invention, an improved die set is provided for forming a multi-compartmented tray from sheet material. The die set includes a female die assembly formed to include at least two mold cavities and a drawing surface bounding said mold cavities, first compartment-former means for pressing sheet material into a selected mold cavity to form a compartment therein, second compartment-former means for pressing sheet material into the remaining mold cavities to form compartments therein, and draw ring means for holding sheet material against the drawing surface during movement of the first and second compartment-former means into their respective mold cavities. The drawing surface provides common drawing regions between each adjacent pair of mold cavities. The draw ring means advantageously acts to provide predetermined tension on the sheet material to reduce the degree of wrinkling of the drawn sheet material substantially.
The first compartment-former means operates to relieve pressure applied to sheet material at a predetermined stage of each molding cycle so that sheet material is able to flow within the selected mold cavity during the drawing process. Such relief aids in reducing the incidence of paper rupture and tearing in the sheet material that might otherwise occur while forming a tray compartment in that cavity.
The first compartment-former means includes a first die for pre-forming the compartment, a second die for finish-forming the compartment, and projection means for moving the first die relative to the second die in a direction toward the cavity from an initial position to a pre-forming position. In such a projected pre-forming position, the first die leads the second die into the cavity to advance sheet material into the mold cavity, thereby pre-forming the tray compartment.
Control means is also provided for disabling the projection means at a predetermined position of the first die in the cavity to permit relative movement between the leading first die and the trailing second die during finish-forming of the tray compartment. Such relative movement allows the first die to relinquish its leading position as the second die finish-forms the tray compartment so that the projection means does not act to apply pressure to sheet material during drawing operations associated with finish-forming of the tray compartment.
In preferred embodiments of the present invention, the second die is formed to include a pressurizable piston chamber. The projection means includes means for pressurizing the piston chamber and piston means for using pressure in excess of a predetermined amount in the piston chamber to apply a predetermined force to the first die. This predetermined force acts to move the first die in relation to the second die to its projected pre-forming position.
In addition, the control means includes position-sensing means for indicating when the leading first die reaches the floor of the mold cavity to define the predetermined position of the first die. The control means also includes valve means responsive to the position-sensing means for venting the piston chamber to reduce the pressure therein below a threshold amount upon arrival of the first die at the predetermined position to permit relative movement between the first and second dies.
In operation, the control means acts to release pressure exerted by the piston means on sheet material trapped between the first die and the floor of the mold cavity so that the only force effectively bearing on the sheet material after disablement of the projection means is the weight of the first die itself. This pressure-release feature advantageously allows sheet material underneath the first die to flow between the first die and the floor of the mold cavity without significantly weakening or otherwise substantially thinning the bottom wall of the tray compartment formed in the mold cavity. Thus, sheet material already in the mold cavity as a result of the pre-forming step carried out using the first die is allowed to flow toward the drawing surface of the female die assembly as new sheet material is drawn into the mold cavity during a subsequent finish-forming step carried out using the second die. Such flow distributes sheet material within the mold cavity to regions experiencing high stress during the drawing process. Such distribution advantageously reduces the likelihood that those highly stressed portions of the sheet material will be stretched beyond elastic limit of the sheet material during formation of a tray compartment in the selected mold cavity. The incidence of sheet material rupturing and tearing is advantageously reduced by use of the novel pressure relief die system in comparison to the amount of damage during compartment formation which occurs using known die systems.
Additional objects, features, and advantages of the invention will become apparent to those skilled in the art upon consideration of the following detailed description of preferred embodiments exemplifying the best mode of carrying out the invention as presently perceived.